1.2 Heat Treatment of Ferrous Metals

1.2 Heat Treatment of Ferrous Metals

Heat treatment is a series of controlled heating, soaking, and cooling operations applied to solid metals to alter their mechanical and physical properties without changing their shape. In aerospace maintenance, heat treatment is used to relieve manufacturing stresses, soften alloys for cold forming or machining, or develop extreme tensile strength and surface hardness in flight-critical components.

Under EASA Part-66 Module 06, maintenance certifying staff must master the iron-carbon equilibrium diagram, understand the kinetics of cooling curves, and distinguish between through-hardening and case-hardening processes.

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The Iron-Carbon Equilibrium Phase Diagram

The iron-iron carbide ($Fe-Fe_3C$) phase diagram maps the equilibrium phases of steel as a function of temperature and carbon concentration (up to 6.67% C by weight, where pure iron carbide or cementite forms).

Temperature (°C)
  1538°C ── Pure Iron Melting Point
         |
  1148°C ── Eutectic Point (4.3% C) ── Ledeburite
         |
   912°C ── A3 Line (Upper Critical Line for Hypoeutectoid)
         |   \ 
         |    \  Austenite (γ) [FCC]
         |     \                 Acm Line (Upper Critical Line for Hypereutectoid)
         |      \               /  (Austenite + Cementite)
   727°C ───────[Eutectoid Point: 0.77% C, 727°C]──────────────── A1 (Lower Critical Line)
         |         /
         |        /  Ferrite (α) + Pearlite  |  Pearlite + Cementite (Fe3C)
    20°C └───────┴──────────────────────────┴─────────────────────────────────────── Carbon (%)
               0.022%                     0.77%                                    2.14%
              (Ferrite)                (Eutectoid)                             (Max Steel)

Key Microstructural Constituents

1. Ferrite (α): Interstitial solid solution of carbon in BCC alpha iron. Extremely low carbon solubility (0.022% at 727°C, dropping to 0.008% at room temperature). Soft, ductile, and magnetic.
2. Austenite (γ): Interstitial solid solution of carbon in FCC gamma iron. Maximum carbon solubility of 2.14% at 1148°C. Ductile, tough, non-magnetic, and exists only above 727°C in unalloyed carbon steels.
3. Cementite ($Fe_3C$): An intermetallic compound containing exactly 6.67% Carbon by weight. Extremely hard (> 800 HV) and brittle, with negligible ductility.
4. Pearlite: A mechanical mixture (eutectoid product) formed by the simultaneous cooperative decomposition of austenite at 727°C (1341°F) in steel containing 0.77% Carbon. Pearlite consists of microscopic, alternating finger-print-like lamellae (plates) of 88% Ferrite and 12% Cementite. It combines good tensile strength with moderate ductility.

Critical Temperatures and Lines

• $A_1$ Line (Lower Critical Temperature, 727°C / 1341°F): The horizontal boundary below which no austenite can stably exist under equilibrium conditions. On heating ($Ac_1$), pearlite begins transforming to austenite at 727°C.
• $A_3$ Line (Upper Critical Temperature for Hypoeutectoid Steels): Slopes downward from 912°C to 727°C as carbon content increases from 0% to 0.77%. Heating hypoeutectoid steel ($< 0.77%\text{ C}$) above $A_3$ ($Ac_3$) fully dissolves proeutectoid ferrite into uniform austenite.
• $A_{cm}$ Line (Upper Critical Temperature for Hypereutectoid Steels): Rises from 727°C to 1148°C as carbon increases from 0.77% to 2.14%. Heating hypereutectoid steel ($> 0.77%\text{ C}$) above $A_{cm}$ ($Ac_{cm}$) completely dissolves boundary cementite networks into austenite.

Terminology Note: In French metallurgy (adopted internationally), 'c' stands for chauffage (heating, e.g., $Ac_1$, $Ac_3$) and 'r' stands for refroidissement (cooling, e.g., $Ar_1$, $Ar_3$). Because of thermal hysteresis, $Ac$ temperatures on heating are always slightly higher than $Ar$ temperatures on cooling.

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Annealing Processes

Annealing encompasses thermal operations designed to soften steel, enhance machinability, relieve internal residual stresses, and refine coarse grain structures.

Full Annealing

Full annealing is applied to forgings and castings. The steel is heated into the single-phase austenite region ($30^\circ\text{C}$ to $50^\circ\text{C}$ above $A_3$ for hypoeutectoid steels), soaked thoroughly (1 hour per 25 mm of thickness), and then cooled at a very slow rate inside the insulated furnace. This slow cooling rate allows carbon atoms ample time to diffuse, forming coarse, widely spaced lamellae of pearlite. The resulting steel possesses lowest possible hardness and yield strength, with maximum elongation.

Spheroidising Annealing

High-carbon steels ($> 0.60%\text{ C}$) such as SAE 52100 bearing steel are exceedingly abrasive to cutting tools in the lamellar pearlite condition. By soaking the steel just below the $A_1$ lower critical line (around 700°C) for 6 to 12 hours, the thin cementite plates break down under surface tension into globular spheres (spheroids). This spheroidal microstructure provides lowest tool wear during turning and milling.

Stress Relieving

Stress relieving operates entirely below the $A_1$ line (550°C to 650°C). Because no phase transformation occurs, the material's heat-treated tensile strength and hardness are unaffected, but lattice elastic strains induced by machining, welding, or heavy cold working are relaxed, preventing post-machining distortion.

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Normalising

Normalising consists of heating steel to 40°C to 60°C above the upper critical line ($A_3$ for hypoeutectoid, $A_{cm}$ for hypereutectoid steels), holding until completely austenitized, and then cooling in still ambient air at room temperature.

Metallurgical Differences: Normalising vs. Full Annealing

• Cooling Rate: Normalising air cooling is considerably faster than the slow furnace cooling of full annealing.
• Grain Size and Morphology: The faster air cool suppresses high-temperature grain growth and forces austenite to transform at lower temperatures, producing very fine lamellar pearlite with uniform, equiaxed grains.
• Mechanical Properties: Normalised steel has higher tensile strength, higher yield point, and greater impact toughness than fully annealed steel, while retaining sufficient ductility for moderate forming.
• Aviation Application: Normalising is standard practice for welded tubular fuselage structures (e.g., SAE 4130 tube assemblies), aircraft forgings, and castings to erase inhomogeneous thermal gradients, refine coarse dendritic cast structures, and establish uniform baseline properties prior to final hardening.

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Hardening of Aircraft Steels

Hardening is the process of heating steel into the austenitic region and cooling it rapidly enough to produce a fully martensitic crystal structure.

Austenitising Parameters

• Hypoeutectoid Steels (< 0.77% C): Heated to 30°C to 50°C above $A_3$. Heating below $A_3$ leaves soft, untransformed proeutectoid ferrite in the structure, causing soft spots and degraded fatigue life.
• Hypereutectoid Steels (> 0.77% C): Heated to 30°C to 50°C above $A_1$ (not above $A_{cm}$). Heating above $A_{cm}$ would cause coarse austenite grain growth and dissolve all cementite, which would cause excessive retained austenite and severe quench cracking. By heating just above $A_1$, hard, wear-resistant undissolved cementite particles remain dispersed within the martensite.

Critical Cooling Velocity and Quenching Media

To form 100% martensite, the steel must be cooled fast enough to avoid the "nose" of the Time-Temperature-Transformation (TTT) or Continuous Cooling Transformation (CCT) curve, where austenite decomposes into pearlite or upper/lower bainite. This threshold is the Critical Cooling Velocity (CCV).

$\text{Severity of Quench: } \text{Brine (10\% NaCl)} > \text{Water} > \text{Polymer} > \text{Quenching Oil} > \text{Still Air}$

The As-Quenched Condition

As-quenched martensite is glass-brittle, highly stressed, and dimensionally unstable. In thick components, transformational volume expansion (martensite has a lower density and larger specific volume than austenite) creates massive internal tensile stresses. As-quenched steel must NEVER be placed into service or left sitting at room temperature; it must be tempered immediately to prevent spontaneous stress cracking.

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Tempering of Hardened Steels

Tempering is the mandatory process of reheating as-quenched martensitic steel to a sub-critical temperature below $A_1$ (between 150°C and 650°C / 300°F and 1200°F), soaking, and cooling at a controlled rate.

Metallurgical Mechanism

During tempering, trapped carbon atoms diffuse out of the strained BCT lattice to form sub-microscopic, finely dispersed iron and alloy carbides, while the tetragonal lattice relaxes into stable Body-Centered Cubic (BCC) ferrite. This aggregate microstructure is tempered martensite.

• Hardness and ultimate tensile strength decrease in a predictable, controlled manner.
• Ductility (% elongation, reduction of area), fracture toughness, and impact resistance increase dramatically.
• Internal quenching stresses are dissipated.

Temperature Regimes and Aircraft Applications

1. Low-Temperature Tempering (150°C–250°C / 300°F–480°F): Relieves residual stresses with negligible loss of hardness. Retains 58–62 HRC. Used for case-hardened gear teeth, bearing races, and cutting tools.
2. High-Temperature Tempering (450°C–650°C / 840°F–1200°F): Develops maximum toughness, high fatigue endurance, and high yield-to-tensile ratio. Yields 32–45 HRC with tensile strengths of 1000–1400 MPa. Standard for landing gear forgings, engine mount bolts, and wing attachment pins.

Temper Embrittlement

Certain alloy steels (especially containing nickel, chromium, and manganese with trace impurities of phosphorus, tin, or antimony) suffer a severe drop in notch toughness if heated within or slowly cooled through the range of 250°C to 400°C (480°F to 750°F) or 350°C to 550°C. This phenomenon, known as temper embrittlement, causes intergranular fracture along prior austenite grain boundaries.

Exam Warning / Common Trap: To avoid temper embrittlement in structural alloy steels (such as 4340), aircraft heat treatment specifications explicitly prohibit tempering in the 250°C–400°C danger band. Furthermore, steels tempered at higher temperatures (above 550°C) must be rapidly cooled (oil or water quenched) through the embrittlement range, rather than slowly cooled in the furnace.

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Case Hardening (Surface Hardening)

Many flight-critical components—such as transmission gears, spline shafts, cam tracks, and piston pins—require an extremely hard, wear-resistant outer surface ("case") coupled with a tough, ductile, shock-absorbing inner core. Case hardening accomplishes this through thermochemical diffusion or localized surface heating.

Carburising

In carburising, low-carbon steel is heated into the austenite region (870°C to 950°C) in a carbon-rich environment. Carbon atoms diffuse into the surface layer, raising surface carbon content to approximately 0.8% to 1.0%. Carburising methods include:

• Pack Carburising: Parts are packed into heat-resistant boxes with hardwood charcoal and barium carbonate ($BaCO_3$) activator.
• Gas Carburising: Parts are placed in a sealed retort furnace with endothermic carrier gas enriched with methane ($CH_4$) or propane ($C_3H_8$).
• Liquid Carburising: Parts are immersed in a molten salt bath containing sodium cyanide ($NaCN$) and barium chloride.

Because the core has low carbon (0.15%), upon subsequent quenching and low-temperature tempering, the outer high-carbon case hardens into wear-resistant martensite (60–62 HRC), while the low-carbon core transforms into low-carbon martensite and ferrite with superior impact toughness.

Nitriding

Nitriding is a thermochemical surface-hardening process performed between 500°C and 550°C (930°F and 1020°F)—completely below the $A_1$ lower critical temperature. The steel is exposed to cracked anhydrous ammonia gas ($NH_3$) in a sealed furnace. The ammonia dissociates:

$2NH_3 \rightarrow 2N\text{ (nascent atomic nitrogen)} + 3H_2$

Atomic nitrogen diffuses into the steel and combines with alloying elements—specifically Aluminum, Chromium, Molybdenum, and Vanadium—to precipitate exceptionally hard, submicroscopic alloy nitrides ($AlN, CrN, Mo_2N$).

Crucial Advantages of Nitriding in Aviation

1. Zero Phase Transformation & No Quenching: Because nitriding is conducted at 500°C–550°C and requires no quench, there are no thermal or transformational volume changes. Components experience virtually zero distortion, allowing parts to be completely ground and finish-machined prior to nitriding.
2. Extreme Surface Hardness: Hardness reaches 65 to 72 HRC (1000 to 1100 HV), far harder than carburised cases.
3. Hot Hardness: Nitrided cases retain their full hardness when heated up to 500°C (932°F), whereas carburised or flame-hardened cases soften permanently above 200°C.
4. High Fatigue Life: The precipitation of nitrides introduces high compressive residual stresses in the surface, elevating fatigue limits by 20% to 40%.

The "White Layer" Trap: Nitriding forms a brittle outermost iron nitride compound layer ($Fe_2N / Fe_4N$) known as the "white layer" (0.005–0.02 mm thick). In high-stress gears, this brittle white layer must be removed by light honing or chemical etching to prevent flaking and spalling in service.

Flame and Induction Surface Hardening

Flame and induction hardening require no change in chemical composition. The part must already be manufactured from a steel with sufficient carbon (0.40% to 0.50% C, such as SAE 4140 or 1045). In induction hardening, high-frequency alternating current passes through a copper inductor coil, generating intense electromagnetic eddy currents that heat the outer surface skin above $A_3$ in seconds. An integral water spray quench immediately cools the surface to martensite, followed by tempering. The interior core remains at ambient temperature, preserving its original tough, ductile properties.